1. Field of the Invention
The present invention relates to a focus detection apparatus having a plurality of focus detection areas.
2. Related Background Art
Traditionally, there has been known a focus detection apparatus having a plurality of focus detection areas in a photographing image plane, which is capable of detecting the focus detection state of its photographing optical system on the basis of the output signals from a plurality of light receiving portions for each of these focus detection areas (as disclosed in Japanese Patent Application Laid-Open No. 2-50115, for example).
The arrangements of the focus detection area of a focus detection apparatus of the kind are illustrated in FIG. 1A and FIG. 1B.
At first in FIG. 1A, there are arranged on a straight line in the longer side direction of the photographing image plane P, three cross type focus detection areas (hereinafter referred to as cross type focus detection area) with two strip type focus detection areas (hereinafter referred to simply as focus detection area) set in the direction rectangular to each other in a photographing image plane P and the cross type focus detection area located in the center of the image plane is set in the center of the photographing image plane P. Further, one of the focus detection areas included in each of the cross type focus detection areas is set in parallel with the longer side direction of the photographing image plane P. Naturally, therefore, the other one of the focus detection areas is arranged in parallel with the shorter side direction of the photographing image plane P.
Next, in FIG. 1B, three cross type focus detection areas are arranged on one straight line in the diagonal direction of the image plane in the photographing image plane P with the cross type focus detection area in the center of the image plane being set in the center of the photographing image plane P. Further, one of focus detection areas included in each of the cross type focus detection areas is set in parallel with the longer side direction of the image plane. Naturally, therefore, the other one of the focus detection areas is arranged in parallel with the shorter side direction of the photographing image plane P.
However, for the above-mentioned conventional focus detection apparatus, there are problems set forth below.
(1) The focus detection areas included respectively in the plural cross type focus detection areas are set in parallel with each other in the conventional apparatus. Therefore, when an object having an edge or line inclined at an angle of almost 45 degrees to each of the focus detection areas is complemented as shown in FIG. 2A and FIG. 2B, the focus detection becomes disabled, resulting in the degradation of the focus detection precision.
(2) When a portrait photograph is taken, the face of an object person where the focusing is desired is positioned slightly out of the center of the photographing image plane as shown in FIG. 3A and FIG. 3B. However, if the cross type focus detection area is set in the center of the image plane, the focus point on the chest portion of the object person is detected and the face is not focused. Yet it is difficult from the viewpoint of the focus detection optical system to set the other cross type focus detection area in a higher place where the face of a person is possibly positioned in taking a portrait photograph while setting one cross type focus detection area in the center of the image plane.
(3) In a conventional apparatus in which a plurality of the cross type focus detection areas are arranged on one straight line, there is also a possibility that the focus detections become disabled in all of the cross type focus detection areas depending on the photographing compositions in an object having horizontal and vertical patterns often existing in the natural world as shown in FIG. 4A or an object with slanting patterns as shown in FIG. 4B. In order to avoid this, it is necessary to set a plurality of focus detection areas so that they are not arranged in one straight line in the photographing image plane. However, this necessitates making the chip size of the photoelectric conversion sensor larger. Hence leading to an increased manufacturing cost.
(4) For the cross type focus detection areas set in the locations other than the center of the photographing image plane, the length of each of the focus detection areas is not defined in consideration of the asymmetry of the focus detection optical system with respect to the optical axis of the photographing optical system. Therefore, depending on an object, the precision of the focus detection is lowered. The detailed description will be given in this respect.
(5) In a focus detection optical system in which the focus detection areas are set in the center of the image plane and other locations, the flexibility in designing the optical system is restrained in the vicinity of the optical axis and locations away from the optical axis of the focus detection optical system because of the integrated formation of the condenser lens and separator lens. Usually, a priority is given to the precision of the focus detection in the center of the image plane. Accordingly, the performance of the focus detection optical system is inferior to the performance thereof in the vicinity of the optical axis. Also, the focus detection optical system is asymmetrical to the plane including the optical axis of the photographing optical system, and a pair of the secondary object images refocused by the focus detection optical system are also asymmetrical. In consideration of these aspects, the pixel pitches, pixel width, pixel slanting direction, and slanting angles of the light receiving portions of the photoelectric conversion sensor are not defined in the conventional focus detection apparatus. Hence lowering the focus detection precision in the focus detection areas other than the center of the image plane.
Now, the detailed descriptions will be given below as to the above-mentioned item (4) and item (5).
FIG. 5 illustrates a setting example of the three focus detection areas while FIG. 6 illustrates a focus detection system for which the three focus detection areas shown in FIG. 5 have been set.
At first, in FIG. 5, reference marks ACH, ALH, and ARH designate the strip type focus detection areas respectively and are set in the center of the photographing image plane P and other locations therein on one straight line in the longer side direction of the image plane.
Next, in FIG. 6, a reference mark MSK designates a viewing mask arranged in the vicinity of the expected image formation plane of the photographing optical system, and by the openings on the viewing mask MSK, each of the focus detection areas ACH, ALH, and ARH shown in FIG. 5 is formed, and FR, FC, FL designate condenser lenses respectively for the focus detection areas ARH, ACH, and ALH. Reference marks SRH1, SRH2, SCH1, SCH2, SLH1, and SLH2 designate separator lenses, and the separator lenses SRH1 and SRH2, SCH1 and SCH2, and SLH1 and SLH2 form the respective pairs for each of the focus detection areas ARH, ACH, and ALH. Further, a reference mark SNS designates a sensor for performing the photoelectric conversion; EH1 and EH2, the pupil areas of a photographing optical system which is not shown; and AX, the optical axis of a photographing optical system.
The primary image of an object formed on the focus detention areas ARH, ACH, ALH through the pupil area EH1 of the photographing optical system is refocused as three secondary objective images on the sensor SNS through the condenser lenses FR, FC, and FL and the separator lenses SRH1, SCH1, and SLH1. Also, the primary image of an object formed on the focus detection areas ARH, ACH, ALH through the pupil area EH2 of the photographing optical system is refocused as three secondary objective images on the sensor SNS through the condenser lenses FR, FC, and FL and the separator lenses SRH2, SCH2, and SLH2.
The deviated amount of the pair of the secondary objective images refocused on the sensor SNS is detected for the detection of a focus adjustment condition of the photographing optical system. However, in the pair of the secondary objective images refocused by the focus detection optical system, there exists an image deviation amount occurring due to the nonfocusing condition of the photographing optical system and an image deviation amount occurring due to the asymmetrical arrangement of the focus detection optical system to the optical axis AX of the photographing optical system. In other words, the focus detection areas set further away from the center of the image plane where the optical axis AX of the photographing optical system runs are more affected by the focus detection system which is asymmetrically arranged with respect to the optical axis AX.
FIGS. 7A, 7B and 7C show the image heights and distortions when the primary image of the object formed on the focus detection area ACH in the center of the image plane shown in FIG. 5 is refocused by the focus detection optical system shown in FIG. 6 on the sensor SNS as a pair of the secondary objective images. FIG. 7A shows the image height and distortion of the secondary objective image refocused by the separator lens SCH1. FIG. 7B shows the image height and distortion of the secondary objective image refocused by the separator lens SCH2. FIG. 7C shows the distortion difference between the pair shown in FIGS. 7A and 7B.
Also, FIGS. 8A, 8B and 8C show the image heights and distortions when the primary image of the object formed on the focus detection areas ALH and ARH, which are positioned in the locations other than the center of the image plane shown in FIG. 5, is refocused by the focus detection optical system shown in FIG. 6 on the sensor SNS as a pair of the secondary objective images. FIG. 8A shows the image height and distortion of the secondary objective image refocused by the separator lens SLH1 or SRH1. FIG. 8B shows the image height and distortion of the secondary objective image refocused by the separator lens SLH2 or SRH2. FIG. 8C shows the distortion difference between the pair shown in FIGS. 8A and 8B.
The distortion difference which presents a problem when the image deviation amount of the secondary objective image is detected for the focus detection is greater in the distortion difference in the focus detection areas ALH and ARH shown in FIG. 8C which are positioned in the locations other than the center of the image plane than the distortion difference in the focus detection area ACH defined in the center of the image plane as in FIG. 7C. In order to obtain the same focus detection precision in the center of the image plane in the detection areas which are positioned in the locations other than the center of the image plane, it is necessary to make the length of the focus detection area ALH and ARH in the locations other than the center of the image plane to be a length WR which is shorter than the length WC of the central focus detection area ACH as shown in FIG. 8C.
Also, as shown in FIG. 9, if the focus detection areas ALV and ARV are set in the direction toward the contacting line of the center of the image plane and the concentric circles even in the focus detection areas in the locations other than the center of the image plane, the asymmetry of the focus detection optical system is modified better than the focus detection areas ALH and ARH defined in the radial directions from the center of the image plane as shown in FIG. 5 although it is still inferior to the distortion difference between the pair of the secondary objective images in the focus detection area ACH set in the center of the image plane. Accordingly, the difference becomes smaller than the distortion difference in the focus detection areas ALV and ARH. As a result, the length of the focus detection areas ALV and ARV can be set longer than the focus detection areas ALH and ARH so as to enable focus detections in a wide range.
FIG. 10 illustrates a focus detection optical system in which the cross type focus detection areas are set in the center of the photographing image plane P and in the locations other than the center thereof. In this respect, the same reference marks are given to the same elements of the focus detection optical system shown in FIG. 6, and the description will be made mostly of the points which differ therefrom. Also, the focus detection area set on the right-hand side of FIG. 10 is symmetrical to the optical axis AX of the photographing optical system. Accordingly, the description will be centered on the focus detection area ALH and ALV set on the left-hand side of FIG. 10.
In FIG. 10, a reference mark FL designates a condenser lens; RX, a diaphragm mask; and RLH1, RLH2, RLV1 and RLV2, the openings of the diaphragm mask RX. The diaphragm mask openings RLH1 and RLH2, and RLV1 and RLV2 form pairs respectively, and the focus detection areas ALH and ALV are respectively set therefor. Reference marks SLH1, SLH2, SLV1 and SLV2 designate separator lenses, and the SLH1 and SLH2, and SLV1 and SLV2 form pairs respectively. The diaphragm mask openings RLH1 and RLH2, and RLV1 and RLV2 are set respectively therefor. Further, reference marks PLH1, PLH2, PLV1 and PLV2 designate the light receiving portions arranged on the sensor SNS for each of the focus detection areas ALH and ALV.
The primary image of an object formed on the focus detection area ALH through the pupil areas EH1 and EH2 of the photographing optical system is refocused on the light receiving portions PLH1 and PLH2 on the sensor SNS as the secondary objective images through the condenser lens FL, diaphragm mask openings RLH1 and RLH2 and separator lenses SLH1 and SLH2. Also, the primary image of an object formed on the focus detection area ALV through the pupil areas EV1 and EV2 of the photographing optical system is refocused on the light receiving portions PLV1 and PLV2 of the sensor SNS as the secondary objective images through the condenser lens FL, diaphragm mask openings RLV1 and RLV2 and separator lenses SLV1 and SLV2. In other words, for the focus detection for the focus detection area ALH set in the radial directions form the center of the image plane in the locations other than the center of the image plane, a light beam passing through the exit pupil areas EH1 and EH2 of the photographing optical system is used. On the other hand, for the focus detection for the focus detection area ALV set in the direction of the contacting line of the center of the image plane and the concentric circles in the locations other than the center of the image plane, a light beam passing through the exit pupil areas EV1 and EV2 of the photographing optical system is used. In this respect, the light beam reaching the points other than the center of the image plane is caused to restrain its light beam not only by the diaphragm of the photographing optical system, but also by the outer diameter of the hood and lens and others.
FIG. 11 shows a substantial shape of the pupil aperture EX viewed from the focus detection areas ALH and ALV set in the locations other than the center of the image plane to the lens side.
As shown in FIG. 11, the aperature is not a circle in the locations other than the center of the image plane and the exit pupil areas EH1, EH2, EV1, and EV2 are set therein. The pupil areas EV1 and EV2 for the focus detection area ALV set in the contacting line of the center of the image plane and concentric circles in the locations other than the center of the image plane are the symmetrical areas to the pupil aperture shape EX. Thus, the light beams having passed these areas have almost the same aberration characteristics. Therefore, the pair of the secondary objective images formed by these light beams are symmetrical, and are suitable for the focus detection. On the other hand, the pupil areas EH1 and EH2 for the focus detection area ALH set in the radial directions from the center of the image plane in the locations other than the center of the image plane are asymmetrical areas to the pupil aperture shape EX. Thus, the light beams having passed these areas have different aberration characteristics. As a result, the secondary objective images formed by these light beams lose their symmetry, and if a focus detection is performed using the secondary objective images in the locations away from the center of the focus detection area ALH, the focus detection error becomes great as described above.
Thus, when the focus detection areas are set in the locations other than the center of the photographing image plane, the length of those focus detection areas should be made shorter than the length of the focus detection area set in the center of the image plane, and further, the length of the focus detection areas set in the radial directions from the center of the image plane should be made shorter than the length of the focus detection area set in the direction of the contacting line of the center of the image plane and concentric circles for performing focus detection, so that the focus detection error should be minimized.